Question

Diketahui suatu deret geometri, suku ke-3 = 45 dan suku ke-5 = 405. Suku ke-9 deret tersebut adalah...

Answer 1

Dik :
U3 = 45
U5 = 405

dit :
U9 = ?

jawab :
Un = a+(n-1)b
U3 = a+(3-1)b = 45
= a+2b = 45
Un = a+(n-1)
U5 = a+(5-1)b = 405
= a+4b = 405

a+4b = 405
a+2b = 45
----------- -
2b = 360
b = 360/2
b = 180
a+b = 45
a+4 = 45
a = 45-4
a = 41

U9 = a+(n-1)b
U9 = 41 + (9-1)180
= 41 + 8 x 180
= 41 + 1440
= 1481